Enumeration of the cells collection. This makes it possible to use Linq-queries.

Array formulas

Pivot tables

Data validation

Conditional formatting

VBA

What is NOT supported by the library (these are the most obvious features)?

Drawing objects

Lines, smart art

Charts Bubble-, Radar-, Stock- and Surface charts.

Embedded Objects

handling of External links (you can reference an external workbook using formulas)

Themes

Chart & Macro worksheets

Form & ActiveX controls

How do I install this library
Unzip the package and copy the EPPlus.dll to a directory of your choice, then simply add a reference to it in your project.

How do I get started?
Download the sample project and step through the different samples in the console application. The samples show most things you can use the library for. There is also a simple web sample that shows different ways to send your generated workbooks to the client.
The sample project is created in Visual Studio 2010.

How do I address a range?
This works pretty much as it works in Excel. Here are a few samples...

worksheet.Cells["A1"].Value = 1; //Set the value of cell A1 to 1
worksheet.Cells[1,1].Value = 1; //Set the value of cell A1 to 1
worksheet.Cells["A1:B3"].Style.NumberFormat.Format = "#,##0"; //Sets the numberformat for a range
worksheet.Cells[1,1,3,2].Style.NumberFormat.Format = "#,##0"; //Same as above,A1:B3
worksheet.Cells["A1:B3,D1:E57"].Style.NumberFormat.Format = "#,##0"; //Sets the numberformat for a range containing two addresses.
worksheet.Cells["A:B"].Style.Font.Bold = true; //Sets font-bold to true for column A & B
worksheet.Cells["1:1,A:A,C3"].Style.Font.Bold = true; //Sets font-bold to true for row 1,column A and cell C3
worksheet.Cells["A:XFD"].Style.Font.Name = "Arial"; //Sets font to Arial for all cells in a worksheet.//Sets the background color for the selected range (default is A1). //A range is selected using the by using the worksheet.Select method
worksheet.Cells.Style.Fill.PatternType = ExcelFillStyle.Solid;
worksheet.Cells.Style.Fill.BackgroundColor.SetColor(Color.LightGreen);

My number formats does not work
If you add numeric data as strings (like the original Excelpackage do), Excel will treat the data as a string and it will not be formated. Do not use the ToString method when setting numeric values.

I get an error that Excel has found unreadable content when I open my generated document?
Check your Formulas and Numberformats. Formulas and numberformats are not validated by the library so if you enter anything wrong the package will be corrupt. Use the English formula names. Formula parameters are separated by commas (,), string parameters use
double quotes (").

worksheet.Cells["A1"].Formula="CONCATENATE(\"string1_\",\"test\")";

Numberformats:
Use culture independent number formats: dot (.) for decimal, comma (thousand).
The easiest way to check a format or a formula is to create an Excel file --> Do the formatting / Add the formulas --> Save it as XLSX ---> Rename the file *.zip. Extract the file and have a look at the number formats in the \xl\styles.xml file and
formulas in \xl\worksheets\sheet#.xml.

I don't know what number format code to use
Do the same procedure as above and look at the formats in the styles.xml file

I have a lot of data I want to load. What should I think of to get the best performance?
If you have more than, let's say 5 000 cells in the sheet and experience performance problems, have this in mind...
Always load cells from top to bottom left to right. That means that if you add a formula on the data you load, it's faster to add it for every row than to add it for a range after you have loaded the data (for ex. this will take a while,Worksheet.Cells"C1:C100000".Formula="A1+B1").
If you want to use a shared formula, like the sample, make sure you create the cells in sequence (use something like worksheet.Cellsrow,forumlaColumn.Value=null to
create the cell. Then you can add the formula after you have finished loading your data.
Avoid using the InsertRow and DeleteRow methods.